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Best mounting practice for 14" SCT to avoid flexture for AP?

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#1 Eric H

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Posted 04 July 2019 - 01:18 PM

I've got a Meade 14" SCT mounted with its standard Losmandy style plate and partial rings sitting on a Paramouunt MX+.

 

My questions are, does the tube flex? By that I mean does the front corrector plate sag in relationship to the main mirror at any time say while performing a meridian flip?

 

Would some kind of more rigid mounting system avoid this, perhaps a Homeyer craddle or something of that nature?

 

Would mounting the top dovetail to connect the front and back help?

 

Any ideas?

 

Thanks all!



#2 bobzeq25

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Posted 04 July 2019 - 01:54 PM

Mounting a top dovetail definitely helped my 6 inch R-C, but that was a pretty flexible scope.  Don't know if yours needs it, but it's just money.  <smile>

 

EDIT.  My advice was based on that specific scope.  jhayes tucson (below) knows infinitely more about C14s.


Edited by bobzeq25, 04 July 2019 - 05:06 PM.


#3 jhayes_tucson

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Posted 04 July 2019 - 04:09 PM

If it's a 14" scope, it flexes.  The key question is by how much relative to the maximum allowable?   Ideally, for high quality imaging under good seeing conditions, you'd like to control the sum of the variance of the pointing errors to less than about (0.25 arc-seconds.)2  The best way to do that is to guide using stars in the image plane itself with OAG or ONAG.  That way slowly varying pointing errors due to mechanical flexure are compensated through guiding.  Mechanical flexure can be a bit more problematic when it comes to pointing the telescope.  Sky modeling can help reduce (or even eliminate) the pointing errors but how well it works may depend on what software you use and how well your scope is aligned to the mount.  Astrophysics supplies software for sky modeling but frankly, it stinks.  The SkyX and other programs may work better so your results may vary.

 

In general, I don't believe in using software to correct primary mechanical errors.  In my view, software correction is better left to correct smaller, higher order errors.  I suggest that you get the mechanics right before relying too much on software to fix things.  I would first check the tightness of all of the tube fasteners to ensure that everything is mechanically very secure.  Then I'd get either tube rings or a high quality dovetail rail to mount to the scope.  I would not use a top rail.  That adds little to the mechanical stiffness of the system and only adds weight where you don't want it.  Mount the OTA on the mount and check the mechanical stability.  If you've done it right, it will feel like it's welded to the DEC axis.  You should not be able to move it, distort it, or get it to vibrate in any way--and that's about the best you can do.  At that point, any residual mechanical flexure should be well compensated for by the guiding system.

 

John


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#4 Spacetravelerx

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Posted 04 July 2019 - 11:20 PM

@Eric H

I am sure there is some flexure in the Meade 14" however I simply have not noticed it in any of my images - my rig is a 14" f/8 ACF on the LX850.  I have found the cradle and dove tail on the 14" f/8 ACF to be incredibly solid.

I do not use an OAG - StarLock's dual guiding works sufficiently well based on all the images I have taken.  I cannot speak for other mounts.

You did not say what kind of Meade 14" SCT you have.  f/10? f/8? A deforked LX200 f/10?

 


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#5 carolinaskies

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Posted 05 July 2019 - 07:13 AM

I've got a Meade 14" SCT mounted with its standard Losmandy style plate and partial rings sitting on a Paramouunt MX+.

 

My questions are, does the tube flex? By that I mean does the front corrector plate sag in relationship to the main mirror at any time say while performing a meridian flip?

 

Would some kind of more rigid mounting system avoid this, perhaps a Homeyer craddle or something of that nature?

 

Would mounting the top dovetail to connect the front and back help?

 

Any ideas?

 

Thanks all!

No, the corrector does NOT sag. 

What SCT owners over the decades have dealt with is mirror 'flop' which is a minute shift of the PRIMARY on the central baffle support tube which it rides due to how it was designed.  The shift distance is extremely small, but when shooting a high FLs some imagers see it with certain smaller SCTs.  However many Meade SCTs come with mirror locks which eliminate this issue. 

The newest F/8 14" does not include mirror locks as it has a more robust crayford bearing system on the central baffle support tube to handle this problem.  

Flexure of the tube system in the F/10 and shorter FL SCTs made by Meade and Celestron is not typically noticeable or measurable except in the most bleeding edge cases where imaging train and target sensitivity is quite high.  In the old film days it was more of a problem when exposures exceeded 30 minutes and more. 

Cassegraine cameras with their larger focal ratios and much longer tubes on the other hand can experience flexure, but they are different beasts from your 14". 

If you are noticing issues with a meridian flip situation the tube and mirror optics is not the first area to check, but rather the imaging train itself.  How is it arranged? How are data/power/control cables moving during a flip, etc.  Even a minute drag on a line can cause a shift.  

FWIW, many people image past the meridian and then initiate the flip before starting on the next imaging run, this gives the opportunity to check focus and eliminate the chance of issue during a flip procedure


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#6 Eric H

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Posted 05 July 2019 - 10:09 AM

Thank you all! Great info!



#7 Stephen Kennedy

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Posted 07 July 2019 - 08:56 PM

From a Physics standpoint, it would seem that any Cassegrain- SCT, RC, CC- would be the type of OTA that would be least susceptible to flexure.  Their compact size would give them a somewhat shorter moment arm than a typical Newtonian and a much shorter moment arm than a small aperture refractor. 


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#8 Whichwayisnorth

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Posted 10 July 2019 - 03:15 PM

You can get rings for it, lock the mirror down, use an external focuser like the Nightcrawler or the Optec Gemini focuser. You'd have a more rigid system.


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#9 jhayes_tucson

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Posted 10 July 2019 - 05:36 PM

From a Physics standpoint, it would seem that any Cassegrain- SCT, RC, CC- would be the type of OTA that would be least susceptible to flexure.  Their compact size would give them a somewhat shorter moment arm than a typical Newtonian and a much shorter moment arm than a small aperture refractor. 

 

Are you sure?  They are indeed shorter but have you considered the optical magnification of the secondary?

 

John


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#10 carolinaskies

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Posted 10 July 2019 - 06:26 PM

Are you sure?  They are indeed shorter but have you considered the optical magnification of the secondary?

 

John

Rigidity is higher the shorter the length between two points in such a system.  Consider that an SCT generally either is supported along the long axis on a GEM/CEM or from the main weight toward 1/4 of the OTA distance on a fork system.  Compare that to a newtonian or refractor whose main weight (objective on refractor, primary mirror on Newtonian) is considerably forward or behind the ring support system in place.  

The 5x spherical/aspherical mirror on an SCT/mct/etc would more lend itself to out of colimation errors than 'sag' as the secondary mirror/corrector is quite light in comparison to an objective cell of a similar sized refractor or primary mirror weight of a newtonian.  Remember that SCT mirrors are thinner AND have a central bore which further reduces weight by comparison.   
 



#11 jhayes_tucson

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Posted 10 July 2019 - 08:33 PM

Rigidity is higher the shorter the length between two points in such a system.  Consider that an SCT generally either is supported along the long axis on a GEM/CEM or from the main weight toward 1/4 of the OTA distance on a fork system.  Compare that to a newtonian or refractor whose main weight (objective on refractor, primary mirror on Newtonian) is considerably forward or behind the ring support system in place.  

The 5x spherical/aspherical mirror on an SCT/mct/etc would more lend itself to out of colimation errors than 'sag' as the secondary mirror/corrector is quite light in comparison to an objective cell of a similar sized refractor or primary mirror weight of a newtonian.  Remember that SCT mirrors are thinner AND have a central bore which further reduces weight by comparison.   
 

The ultimate issue is how much can the image move in the image plane due simply to mechanical flexure.  For a C14 under 1"-2" skies, you want to control the angular position of the image to better than 0.20" - 0.25" rms and at that level, pretty much everything is made of rubber.  I haven't done a calculation to compare a SCT to a Newtonian with the same first order properties so maybe the Newtonian flexes more but in either case, mechanical flexure will be large enough that you have to guide through the scope to counter the effects of image motion due to flexure as the scope tracks.

 

John


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#12 carolinaskies

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Posted 11 July 2019 - 08:27 AM

The ultimate issue is how much can the image move in the image plane due simply to mechanical flexure.  For a C14 under 1"-2" skies, you want to control the angular position of the image to better than 0.20" - 0.25" rms and at that level, pretty much everything is made of rubber.  I haven't done a calculation to compare a SCT to a Newtonian with the same first order properties so maybe the Newtonian flexes more but in either case, mechanical flexure will be large enough that you have to guide through the scope to counter the effects of image motion due to flexure as the scope tracks.

 

John

Guiding due to MOUNT deflection(mechanical flexure) (ie periodic error) is more primary a concern than OTA flex at sidereal rate.  With PEC dialed out next comes perceived change in image from atmosphere, and while .2-.25 would be ideal, atmospheric conditions can make that goal unobtainable in many places.   Least would be concern about flex due to OTA design.  

In my experience flexure of the mount causing image shift is far more serious than believing the OTA is somehow sagging.  More often the discussion is external guide scope delta change vs OTA delta change.  Using an OAG or OOAG would mitigate that problem IF the individual felt their system had enough backfocus adjustment to make use of such systems and achieve their goals.   Yet an external guide system is more than adequate if proper attention is paid to selection of components and considerations of where the delta change might occur in the system.   Those of us who guided by eye in the film days can attest to how much better images are today with guide cameras used both in OAG and Guidescope configurations.   The key is understanding the FOV equivalence between guide and main scope and any atmospherics.  Todays more sensitive guide cameras can pick up the delta change much quicker making even a piggyback guider capable of handling big SCTs.  In fact it becomes necessary if the target FOV through the OTA contains no reasonable guide stars to rely on a wider angle guider.  


  


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#13 jhayes_tucson

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Posted 11 July 2019 - 02:55 PM

Guiding due to MOUNT deflection(mechanical flexure) (ie periodic error) is more primary a concern than OTA flex at sidereal rate.  With PEC dialed out next comes perceived change in image from atmosphere, and while .2-.25 would be ideal, atmospheric conditions can make that goal unobtainable in many places.   Least would be concern about flex due to OTA design.  

In my experience flexure of the mount causing image shift is far more serious than believing the OTA is somehow sagging.  More often the discussion is external guide scope delta change vs OTA delta change.  Using an OAG or OOAG would mitigate that problem IF the individual felt their system had enough backfocus adjustment to make use of such systems and achieve their goals.   Yet an external guide system is more than adequate if proper attention is paid to selection of components and considerations of where the delta change might occur in the system.   Those of us who guided by eye in the film days can attest to how much better images are today with guide cameras used both in OAG and Guidescope configurations.   The key is understanding the FOV equivalence between guide and main scope and any atmospherics.  Todays more sensitive guide cameras can pick up the delta change much quicker making even a piggyback guider capable of handling big SCTs.  In fact it becomes necessary if the target FOV through the OTA contains no reasonable guide stars to rely on a wider angle guider.  


  

 

That sure hasn't been my experience with the 4x C14s that I've imaged with--on two different mounts.  My current mount is an AP1600 with encoders and the primary factor that limits pointing accuracy is mechanical flexure due to the OTA and its internal components.  Here's how I know that:  You can easily assess gross flexure by doing a polar field rotation.  Mount a small, solid refractor on your mount, point at the pole and run the mount to the east side of the pier.  Then open the shutter and run the RA to the west side of the pier.  If the trails are not circular, there's flexure in your mount; otherwise it's good.  Now mount your SCT and do the same thing.  If your mount is good, you'll see on a large scale how much flexure is directly related to the OTA.  I've attached a PFR for my C14 system, which is on a good mount that's rated for about 3x the weight that it actually carries.  Look carefully at the trailed stars and you'll see that the system is clearly flexing as it moves through the RA angle.  This turns out to be the limiting factor for pointing errors even when atmospheric correction is applied to the encoder controlled position.  In general, this scope is only good to about 0.3 degrees over the whole sky--and that's all due to mechanical flexure.  This scope is pretty bad and some might be better; but, none are perfect--particularly when you look at any scale less than an arc-second.  

 

An external guide scope will indeed work with these scopes, but you'll never achieve very good results with any exposure longer than a minute (or maybe two)--and even then, the performance will be hit and miss.  In order to achieve FWHM stars below about 5", you have to guide through the telescope itself--and that's always due to mechanical flexure between the two scopes.  Keep in mind that mechanical flexure of only 0.25" across the length of a C14 is only about 34 millionths of an inch (0.9 microns or slightly less than 2 wavelengths of light.)  You can't hardly touch that tube without deflecting it by that much!  With my ONAG system, I regularly achieve 0.25" guiding with FWHM below 2" and I've seen as good as 1" when the conditions permit using 20 minute exposures.  That level of performance is impossible using a guide scope.

 

John

 

 

PS  I too started guiding film by eye in the late 60's and it was terrible by today's standard.

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#14 carolinaskies

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Posted 11 July 2019 - 09:01 PM

That sure hasn't been my experience with the 4x C14s that I've imaged with--on two different mounts.  My current mount is an AP1600 with encoders and the primary factor that limits pointing accuracy is mechanical flexure due to the OTA and its internal components.  Here's how I know that:  You can easily assess gross flexure by doing a polar field rotation.  Mount a small, solid refractor on your mount, point at the pole and run the mount to the east side of the pier.  Then open the shutter and run the RA to the west side of the pier.  If the trails are not circular, there's flexure in your mount; otherwise it's good.  Now mount your SCT and do the same thing.  If your mount is good, you'll see on a large scale how much flexure is directly related to the OTA.  I've attached a PFR for my C14 system, which is on a good mount that's rated for about 3x the weight that it actually carries.  Look carefully at the trailed stars and you'll see that the system is clearly flexing as it moves through the RA angle.  This turns out to be the limiting factor for pointing errors even when atmospheric correction is applied to the encoder controlled position.  In general, this scope is only good to about 0.3 degrees over the whole sky--and that's all due to mechanical flexure.  This scope is pretty bad and some might be better; but, none are perfect--particularly when you look at any scale less than an arc-second.  

 

An external guide scope will indeed work with these scopes, but you'll never achieve very good results with any exposure longer than a minute (or maybe two)--and even then, the performance will be hit and miss.  In order to achieve FWHM stars below about 5", you have to guide through the telescope itself--and that's always due to mechanical flexure between the two scopes.  Keep in mind that mechanical flexure of only 0.25" across the length of a C14 is only about 34 millionths of an inch (0.9 microns or slightly less than 2 wavelengths of light.)  You can't hardly touch that tube without deflecting it by that much!  With my ONAG system, I regularly achieve 0.25" guiding with FWHM below 2" and I've seen as good as 1" when the conditions permit using 20 minute exposures.  That level of performance is impossible using a guide scope.

 

John

 

 

PS  I too started guiding film by eye in the late 60's and it was terrible by today's standard.

I believe that error is not internal component delta change.  The nature of suspended weight equatorial mounts is the change across meridian. You are no longer measuring the same deflection across the RA as you have rotated the DEC axis 180* to reorient the OTA.  Therefore you have changed the system. In a perfect world on paper there is no change. But in the real world even with an AP1600 this change is very real at long focal lengths.  

This is why forks are superior to GEMs, E-W tracking a target there isn't going to be flexure caused by reorientation. This is also why the majority of pure science grade mounts are fork designs or are designed to not require meridian flips from E-W horizon.   Invest in a Planewave and toss out the AP1600, you'll never have to deal with your phantom flexure. 



#15 jhayes_tucson

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Posted 11 July 2019 - 11:13 PM

I believe that error is not internal component delta change.  The nature of suspended weight equatorial mounts is the change across meridian. You are no longer measuring the same deflection across the RA as you have rotated the DEC axis 180* to reorient the OTA.  Therefore you have changed the system. In a perfect world on paper there is no change. But in the real world even with an AP1600 this change is very real at long focal lengths.  

This is why forks are superior to GEMs, E-W tracking a target there isn't going to be flexure caused by reorientation. This is also why the majority of pure science grade mounts are fork designs or are designed to not require meridian flips from E-W horizon.   Invest in a Planewave and toss out the AP1600, you'll never have to deal with your phantom flexure. 

 

Paul,

First, I'm sorry but I don't understand what you are saying so maybe you could draw me a picture.  If you believe " that error is not internal component delta change," what do you think is causing it and what evidence do you have to support your belief?  It seems like you are talking about a meridian flip; but, A) That data set did not involve a meridian flip and B) I don't know what that has to do with mechanical flexure relative to the mount vs the OTA.  Mechanical flexure in the whole system (mount+OTA) can masquerade as cone error but I'm not sure if that's what you are taking about.  To be clear, I've shimmed my system to minimize cone error.

 

The PFR image I posted clearly shows flexure within the OTA or with the OTA mounting structure.  I built the OTA and the dovetail support system on this scope and I have a pretty good idea of what might be flexing so I'm not surprised that this particular OTA flexes quite a bit.  It needed one more design iteration but that wasn't possible so I put it into service anyway.  Still data from this scope provides a good demonstration of the problem.  I've seen flexure with the other C14s that I've used as well but it hasn't been quite as bad as with this scope. If you still don't believe in OTA flexure, just put a guide scope on your 16" Meade and show me some images using 20 minute exposures with 1" - 1.7" FWHM stars.  I think that it might be more instructive to discuss this issue while looking at some real data.

 

Second, I actually do have a PW scope + L-series mount on order so we'll see how that works once I get it going sometime next year.  Since all of the optical components are fixed in a truss type structure, I expect mechanical flexure between the components to be relatively small, but still not zero.  That's why I'll still be guiding through the optical system using an ONAG guider.

 

John



#16 Stephen Kennedy

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Posted 13 July 2019 - 01:48 PM

Guiding due to MOUNT deflection(mechanical flexure) (ie periodic error) is more primary a concern than OTA flex at sidereal rate.  With PEC dialed out next comes perceived change in image from atmosphere, and while .2-.25 would be ideal, atmospheric conditions can make that goal unobtainable in many places.   Least would be concern about flex due to OTA design.  

In my experience flexure of the mount causing image shift is far more serious than believing the OTA is somehow sagging.  More often the discussion is external guide scope delta change vs OTA delta change.  Using an OAG or OOAG would mitigate that problem IF the individual felt their system had enough backfocus adjustment to make use of such systems and achieve their goals.   Yet an external guide system is more than adequate if proper attention is paid to selection of components and considerations of where the delta change might occur in the system.   Those of us who guided by eye in the film days can attest to how much better images are today with guide cameras used both in OAG and Guidescope configurations.   The key is understanding the FOV equivalence between guide and main scope and any atmospherics.  Todays more sensitive guide cameras can pick up the delta change much quicker making even a piggyback guider capable of handling big SCTs.  In fact it becomes necessary if the target FOV through the OTA contains no reasonable guide stars to rely on a wider angle guider.  


  

I started imaging with film and a Nikon SLR in the 1980s and I have to say that flexure of the equipment was nowhere near the top of my list of things to be concerned about.  Focusing alone was a major difficulty that required a lot of precise measurements and some calculations and a fair amount of luck.  You could not just take exposures and focus by iteration since you could not see the image until the film was developed and every time you pushed the shutter release cable you burned up another frame of expensive film and with only 12 or 24 exposures on a role you had to put in a new roll of film which is not always easy during the day let alone in complete darkness.  I eventually stopped imaging with film because of its complexity and the need for a lot of things to go right and nothing could go wrong to get images that were not as good as I could see through the eyepiece.  I did not try to image again until 2013 when I received a Canon T3 DSLR for Christmas.  It is amusing some times to go to the Imaging Forum and listen read what people who started imaging in the last few years write about how difficult, complicated and hard to learn AP is.  When I put that T3 on my telescope, focused with live view and instantly saw the perfectly focused image on live view I thought  "Could astrophotography really be this easy now?" 



#17 jhayes_tucson

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Posted 13 July 2019 - 09:09 PM

I started imaging with film and a Nikon SLR in the 1980s and I have to say that flexure of the equipment was nowhere near the top of my list of things to be concerned about.  Focusing alone was a major difficulty that required a lot of precise measurements and some calculations and a fair amount of luck.  You could not just take exposures and focus by iteration since you could not see the image until the film was developed and every time you pushed the shutter release cable you burned up another frame of expensive film and with only 12 or 24 exposures on a role you had to put in a new roll of film which is not always easy during the day let alone in complete darkness.  I eventually stopped imaging with film because of its complexity and the need for a lot of things to go right and nothing could go wrong to get images that were not as good as I could see through the eyepiece.  I did not try to image again until 2013 when I received a Canon T3 DSLR for Christmas.  It is amusing some times to go to the Imaging Forum and listen read what people who started imaging in the last few years write about how difficult, complicated and hard to learn AP is.  When I put that T3 on my telescope, focused with live view and instantly saw the perfectly focused image on live view I thought  "Could astrophotography really be this easy now?" 

 

Hahaha...yeah I started with film back in the late 60's and it was indeed a whole different game.  However, the answer to your last question is a resounding "No!"  Yes, back in the film days achieving a level of focus equivalent to what you can get now using live view was hard; however today, simple raw view focusing is hopelessly inadequate as a repeatable and accurate way to focus.  Adding a Bahtinov mask to live view greatly improves things and that's part of the solution for short exposures.  If you want to maintain really precise focus over a long session, you need some sort of automated focusing system.  It all comes down to what kind of results you expect.  Garden variety imaging that produces results about as good as you ever got with film isn't that hard.  If you want to take it to the next level, things can get a lot more challenging.

 

John


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